Well, you gotta clean out all the seam and joints the best you can. There are many types of good sealants, silicone based ones not included. I like Acyl R, the same stuff I saw at Jackson Center. Then apply a small bead at the joints. Acyl R will wick in a bit.
It is tedious work. Rivets can work loose and leak too. Captain Trollys wicking seam sealer seems to work for those.
After you think you have it sealed up, get a high volume fan and use a "hillbilly" SelTec method of pressurizing the trailer, maybe through an outside storage door. Soap the seams and be surprised at all the leaks. Then try to seal up the leaking spots.
Airstreams are infamous leakers. The moisture rots the subfloor. Diligent leak detection with a moisture meter is an important maintenance activity.